The invention relates to apparatus for drawing a monocrystal from a crucible by means of a coilable pulling element, and in particular by means of a cable. The end of the cable which is arranged above the crucible is secured to a crystal nucleus holder which is connected to a lifting drive means which is arranged on a platform rotatable about the pulling element and comprises a driven horizontal shaft having a coiling drum which is longitudinally displaceable on this shaft and onto which the pulling element can be tangentially coiled.
Since it has already been used in public, apparatus of the above-described kind forms part of the prior art. However, the known apparatus has only one drive motor for the lifting drive means, so that the setting of greatly differing vertical speeds of the crystal-nucleus holder is possible only by way of the motor speed. The precision in the control of such motors, which are exclusively controllable direct-current motors, is, for example, 2.0% of 3000 revolutions per minute in terms of the final speed. The resultant deviations of .+-.6 r.p.m. are still obtained, however, even at low speeds, so that in the case of a control range of 1:1000 down to 3 r.p.m., the relative error would increase to 200%. This error is extremely undesirable in the crystal-drawing process which is carried out only at low speeds of revolution. For this purpose, the known drive has been so designed that the change in speed is only 1:100. The maximum lifting speed is thus limited to approximately 20 cm/min, for example, a value that is too low for a high-speed movement.
For the purpose of setting stepped speeds, it would be theoretically feasible to use motors change-over gears. However, these possible means are precluded, since the gears that might be considered for the purpose tend to vibrate. However, vibrations in the drive system have an effect which could be harmful in the production of monocrystals, since such production is possible only if the drawing process is free from trouble. Furthermore, in the known apparatus, the vertical speed of the crystal-nucleus holder must be variable over a wide range for control purposes and/or for adapting the apparatus to suit various types of crystal, and this of necessity is manifested by a change in speed of the lifting drive motor. Motors with commutatable poles cannot as a rule be used for such a purpose, since they have rigid speed graduation.